Solat has 2 forms - mandatory (fardh/wajib) and sunat (recommended/optional).
The mandatory solat must be performed according to strictly defined
rules at specified times., while the sunnat solat can be performed
anytime except at some specific forbidden time such as after asr and before
maghrib.

Dua:-

As for dua one may "supplicate" God at any time and in any circumstances
without any pattern or formula. Supplications are strictly voluntary and
"free". Some of the most intimate glimpses of the pious Muslim soul are
to be found in supplications!

Wirid:-

Wirid involved recitation of Quranic formulae or one or more of the
Names of God - like supplication these are voluntary, although they
are not so "free" since they follow a set of patterns, like the canonical
solat, which must be in Arabic.

Dzikr:-

While Wirid may be performed by any pious muslim, dhizkr are recited
almost exclusively by the Sufis.

Although supplications left by the great saints of early Islam are of
the type of "free prayer", invariably they have one element in common:

since they are recited in Arabic (although they can be made in any language),
they are largely inspired by Quranic images and incorporate Quranic
verses and formulae. Also they are rhythmic and very often - employ
rhymed prose (saj').